Loop antenna with negative resistance element at terminating gap



Aptil29,1969 R.T.;L.\EITNER 3,441,

LOOP ANTENNA WITH NEGATIVE RESISTANCE ELEMENT AT TERMINATIN GGAP Filed Aug. 19', 1965 INVENTOR ATTORNEY United States Patent US. Cl. 343-701 15 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a loop antenna having a feed gap and an impedance terminating tap. The impedance at said terminating gap is negative and matched in absolute value to the impedance of said antenna.

This invention relates to antennas and more particularly it relates to loop antennas.

A principal object of the invention is to provide a highly efiicient broad-banding loop antenna of relatively small physical dimensional configuration.

Another object is to provide a novel loop antenna having a feed input gap and a terminating impedance gap displaced from or remote from the feed gap which impedance gap includes a special terminating resistance to improve the directional properties as well as the bandwidth properties of the antenna.

A further object is to provide a loop antenna having an impedance terminating gap displaced from the usual feed input gap, and wherein the terminating gap includes an active element such as a tunnel diode having predetermined negative resistance characteristics which cooperate with the loop to improve its efi'iciency and directional characteristics.

A further object is to provide a dual loop antenna including a pair of specially designed negative resistance impedances so located within the loops and with respect to a common junction, to provide a bilateral directional characteristic.

A still further object relates to a dual loop antenna which includes a pair of negative resistance elements such as tunnel diodes in the respective loops, which are connected together to provide a pseudo-passive repeater.

A feature of the invention relates to a highly efficient and broad-banding loop antenna of small dimensional configuration which renders it especially suitable for confined spaces or for indoor use such as in television receivers, FM receivers, and the like.

Another feature relates to a novel loop antenna of relatively small physical size which includes an active negative resistance element such as a tunnel diode, to impart unidirectional radiation characteristics over a comparatively broad frequency band, and which has a substantially fiat VSWR or substantially flat input impedance over the broad band, and is also substantially free from sharp resonances within the band.

Another feature relates to a highly efficient broadbanding loop antenna which includes an active negative resistance element such as a tunnel diode displaced from or remote from the loop input feed gap which improves the radiation pattern and beaming characteristics of the antenna.

A still further feature relates to the novel organization, arrangement and relative location and interconnection of parts which cooperate to provide an improved loop antenna.

In the drawing which shows certain preferred embodiments,

FIG. 1 is a schematic diagram of a loop antenna embodying the invention;

3,441,935 Patented Apr. 29, 1969 FIG. 2 is a schematic diagram of a modification of the invention;

FIG. 3 is a typical antenna radiation field pattern explanatory of the invention.

In certain of the communications arts, for example, in the television field, it is desirable to provide a receiving antenna which is of relatively small physical size and yet has high gain, unidirectional characteristics and uniform input impedance and uniform VSWR for the entire television band, for example all the assigned VHF television channels.

'I-Ieretofore various forms of loop antennas and half wave dipoles have been used for television reception. In general, however, such antennas have required considerable space, and especially in the case of the half wave antennas. While an obvious expedient is to use a loop of small physical size, in general, such reduction in size also involves a reduction in the antenna efiiciency, and does not enable a constant input impedance or a constant VSWR to be obtained.

Furthermore, even such small size loops are not unidirectional and the standard radiation pattern is a figure of eight in the plane perpendicular to the loop and omnidirectional in the plane of the loop. I have found that it is possible to reduce the physical size of the loop antenna, and by including a specially designed terminating impedance within the loop, at a point displaced from and preferably opposite to the usual input feed gap, the desired directional and highly efficient antenna characteristics can be obtained.

While it has been proposed heretofore to include in an antenna such as a dipole, an active amplifying element such as a solid state amplifier, such inclusion has been merely to achieve local amplification of the signal picked up directly by the antenna.

In other words, wherever the prior art has employed such an amplifier as part of an antenna, it has been done merely to increase the amplification efiiciency by using the electrical constants of the antenna arms as part of a tuned tank circuit for the amplifier which thereupon becomes a tuned amplifier, an example of such a tuned amplifier dipole antenna is that disclosed in United States Patent No. 3,098,973. The present invention does not rely upon the antenna arms as forming part of a tuned ampli fier but rather they form a transmission line which has a specially designed terminating impedance in the form of an active negative resistance device such as a tunnel diode. In other words, the loop is provided with the usual input feed gap and with another gap displaced from the feed gap and preferably opposite to it, which other gap is a terminating impedance gap wherein is connected an active negative resistance device such as a tunnel diode. Thus the two halves of the loop can be considered as the conductors of a lossy transmission line having a substantially fiat input impedance characteristic and a substantially flat VSWR. The loss component is that which is associated with antenna radiation. The net result is an antenna which has the desired unidirectional characteristics and which can be loaded at its input feed gap with an attenuating pad so that the antenna exhibits high gain over a broad band of frequencies.

Referring to FIG. 1, the antenna comprises a conductive loop L which merely for simplicity, is shown of circular contour and constituted of the two semicircular conductors 10, 11. ([t will be understood that the loop instead of being circular, may be square, rectangular, or any other desirable geometric contour. The loop has an input feed gap across whose terminals 12, 13 is connected any suitable attenuating pad P comprised for example of series resistors 14, 15 and shunt resistors 16, 17. The terminals 18 and 19 of the pad can be connected to the usual input terminals of a television receiver or similar device. Remote from the input feed gap of the antenna is a terminating gap having terminals 20', 21. Connected directly across the terminals 20', 21 is a tunnel diode 22, in series with a resistor 23.

While the invention is not limited to any particular kind of tunnel diode, it is preferably a tunnel diode of the germanium type. Preferably the tunnel dioxide should be of a type having a very low junction capacity C This permits the use of a positive shunting resistor 24 which has the effect of controlling the value of the composite negative resistance of the tunnel diode circuit and thus maximizing the antenna gain with optimum amplification stability. In order that the tunnel diode may have the necessary stable amplification properties while providing the necessary negative resistance characteristics, the load impedance across the terminals '20, 21 must be such that the net positive resistance as seen by the tunnel diode 22 is less than the tunnel diode negative resistance and the total reactance of the circuit including the reactive components of the tunnel diode must be at resonance at the operating frequency of the antenna. Thus the resistor 24 can be chosen to resistively tune the circuit and accommodate variations in resistance presented across terminals 20, 21 as a function of varying receiver characteristics and varying antenna characteristics with frequency. In a similar manner the inductance 30 can be chosen to reactively tune the circuit and accommodate variations in reactance presented across terminals 20, 21 as a function of varying receiver characteristics and varying antenna characteristics with frequency. Capacitor 29 is used in the circuit when the operating frequency is in the range of 54 to 110 me. where it cooperates with inductance 30 in performing the reactance tuning. This results in an optimum termination from the viewpoint of antenna gain and full radiation pattern. In order to bias the diode 22 there is provided a suitable battery or direct current supply 25 which is connected across resistor 23 in series with an adjustable resistor 26 and an inductance 27 and switch 28.

The radiation pattern of the antenna is a function of the impedance connected across gap 20, 21. For example if this impedance is zero (20, 21 short circuited) the antenna would present a typical loop pattern which is a figure of eight in the plane perpendicular to the loop and a circle in the plane of the loop. If gap 20, 21 is open circuited the antenna would present a typical short dipole pattern which is a figure of eight in the plane of the loop and a circle in the plane perpendicular to the loop. It can be shown that the mathematical addition of a loop and dipole pattern results in pattern of cardioid shape in both planes. Therefore if gap 20, 21 is connected by an impedance of some value between zero and infinity the components of loops and dipole pattern are such as to result in a pattern of cardioid shape in both planes. The required impedance for this pattern characteristic is identical with the average characteristic impedance of the antenna looking at it as a section of short lossy transmission line. The actual value for the geometry and frequencies under consideration is nearly 425 ohms. This case is represented in FIG. 3 where gap 20, 21 is terminated in positive resistance 33 of 425 ohms. The resulting radiation pattern is represented by 34 and is a cardioid shape in the plane of the loop as shown, and also in the plane perpendicular to the loop. The basic disadvantages of this arrangement are the losses associated with such a relatively high resistance when compared with the radiation resistance of the antenna. In other words the antenna possesses good directive gain due to its pattern characteristics but low effective gain due to resistor losses. The invention provides a negative impedance termination across 20, 21 thereby overcoming the resistance losses while maintaining the necessary conditions for a cardioid pattern. In FIG. 3 if the positive resistance 33 of 425 ohms is replaced with a negative resistance of 425 ohms (tunnel diode) there results the same cardioid pattern except directed oppositely as represented by 35.

While I do not wish to be restricted to any theory as to how the antenna possesses its high gain and broadbanding characteristics, the foregoing appears to be a logical explanation. Regardless of the theory of operation, it has been found that by using the tunnel diode negative resistance termination as described the gain of the antenna over an equivalent antenna with a positive resistance termination across gap 20, 21 can be as much as 27 db while also obtaining the desired unidirectional radiation field pattern. The presence of the 12 db pad P composed of '14, 15, .16 and 17 reduces the effective gain to 15 db. The function of the pad is one of smoothing of the impedance which the receiver presents to the antenna not only at the frequency to which the receiver is tuned but at all frequencies for which the tunnel diode presents a negative resistance characteristic.

While the invention is not limited to any particular to produce the desired results for operation over the entire television band (channels 113), the various elements were as follows:

P=l2 db pad attenuator 300 ohms (receiver side) to 425 ohms (antenna side) transformation 17:438 ohms, 14:835 ohms, 1 6:332 ohms 15:5000 ohms, 23:50 ohms, 24:600 to 20K ohms 27:20 microhenries, 30:.08 to .65 microhenry 29:20 ,a f. at frequencies of 54 to me. and 0 ,u tf. at

frequencies of 174 to 216 me.

28:switch, 25:9 V. DC. bias supply 10:antenna loop conductor, loop size:l5" diameter,

conductor size:%" diameter 22=tunnel diode with approximately the following characteristics: R :425 ohms-110%, C =1.5 ,LLMf. max., 11 :5 ohms max., L =.3 microhenry max.

The invention is also capable of being used as a bilateral or pseudo-passive repeater. Thus as shown in FIG. 2, it is composed of a pair of similar conductive loops L-A-L-B. The parts of FIG. 2 which are similar to those of FIG. I bear the same designation numerals but with the respective suffixes A or B.

For explanatory purposes, let it be assumed that the wave energy to be retransmitted is received in the direction of the arrow 31 and is to be retransmitted in the direction of the arrow 32. The gap 12A-13A is located opposite to the termination gap 20A-21A and loop L-A and across that gap is connected a negative resistance network including the tunnel diode 22A and the remaining elements 23A-30A. Likewise, the gap 20B-21B for the loop L-B has connected therecross a similar negative resistance network including the tunnel diode 22B and the remaining elements 23B-30B. The gaps 12A-13A and 12B-13B are interconnected by an attenuation pad P similar to the corresponding pad P of FIG. 1. I have found that with this combination of loops the negative resistance termination networks the received energy can be retransmitted with a substantial net gain in the direction of the transmission. Furthermore, the system of FIG. 2 is directionally sensitive for reception in one direction and directionally sensitive for retransmission in the same direction. In other words, the loop L-A has a unidirectional radiation field pattern in the direction of the received energy and the loop L-B has a unidirectional field radiation pattern in the direction of the retransmitted energy.

While certain specific embodiments and dimensions and values of components have been referred to herein, it will be understood that they are given merely as illustrative and not by way of limitation on the inventive concept.

What is claimed is:

1. A loop antenna having a pair of separated gaps one of which is a feed gap and the other an impedance termination gap, and a circuit having a negative resistance characteristic connected across said other gap.

2. A loop antenna according to claim 1 in which said circuit includes an active element having a negative resistance characteristic.

3. A loop antenna according to claim 2 in which the effective negative resistance at said impedance termination gap is of an absolute value approximately equal to the impedance of said antenna at said gap, whereby a cardioid pattern results in a direction opposite to that of an equal positive resistance at said terminating gap.

4. A loop antenna according to claim 3 in which said active element is a tunnel diode.

5. A loop antenna according to claim 4 in which said tunnel diode is of a kind having low junction capacity, and a positive resistance is connected in shunt to said diode across said other gap.

6. A loop antenna according to claim 5 in which said positive resistance is adjustable to control the composite negative resistance across said gap.

7. A loop antenna according to claim 6 in which an adjustable biasing circuit is connected to said tunnel diode to bias it to a point of stable amplification.

8. A loop antenna according to claim 7 in which a reactive tuning element is connected across said other gap to render the circuit across said other gap resonant within the operating frequency band of the antenna.

9. An antenna comprising a pair of conductors shaped to form a transmission line as a loop, one end of said line constituting a feed gap, the other end of the line constituting an impedance termination gap, said antenna normally having a bidirectional radiation field pattern characteristic, and a negative impedance circuit connected across said termination gap, said circuit including a tunnel diode, means to bias said diode to a point of stable amplification, and positive resistance means also connected to said gap to render said circuit resonant within the operating frequency band of the antenna.

10. The antenna of claim 9 in which said positive impedance is variable.

11. In combination, a pair of conductive loops each having a feed gap and an impedance gap, means interconnecting the feed gaps of the two loops, a negative resistance circuit connected across the impedance gap of one loop, and a separate negative impedance circuit connected across the impedance gap of the other loop.

12. The combination according to claim 11 in which each of said negative impedance circuits includes a tunnel diode.

13. The combination according to claim 12 in which said feed gaps of the two loops are interconnected through an attenuation pad.

14. A passive repeater for receiving wave energy in one direction and retransmitting it in the same direction, comprising a pair of loop antenna elements each element having a feed gap and an impedance gap, means interconnecting the feed gaps of both loops and a negative resistance circuit connected across each of the said impedance gaps and each including a tunnel diode.

15. A loop antenna having a pair of separated gaps, one of which is a feed gap and the other an impedance termination gap, said loop normally having a bidirectional radiation field pattern, and a predetermined negative terminating impedance element connected across said other gap to render the said field pattern unidirectional, and an attenuation pad coupled to said feed gap, whereby said antenna exhibits relatively wide-band, high gain characteristics.

References Cited STATES PATENTS ELI LI'EBERMAN, Primary Examiner.

U.S. Cl. X.R. 343742, 744 

